The present invention pertains generally to Humidity Sensors and more particularly to relative humidity detector systems and methods for increasing the calibration periods of relative humidity detector systems. Conventional humidity sensors utilize an aluminum layer which is partially anodized on its upper surface prior to deposition of the top electrode. The upper electrode is sufficiently thin to allow passage of water molecules to the partially anodized aluminum layer. The partially anodized aluminum layer changes resistance and capacitance when employed in a conventional ac stabilization circuit.
Although conventional humidity sensors have been developed to provide good response, slow anodization of the aluminum base material causes progressive impedance and capacitive change in the sensor due to exposure to moisture and residual acid remaining in the Al.sub.2 O.sub.3 layer after the initial anodization process, especially when subjected to an ac current as utilized in conventional humidity sensor circuitry. As a result, the humidity detecting circuitry must be continually adjusted for proper operation throughout the life of the device. Even more seriously, use of the device in high temperature environments causes rapid aging even though appreciable amounts of moisture may not be present.
Attempts to overcome these problems by methods such as pre-aging the sensor, as disclosed in the prior art, have had limited success due to the non-transient nature of the slow anodization process.
Another attempt to overcome these problems is disclosed in U.S. Pat. No. 4,143,177 issued Mar. 6, 1979 to Kovac et al wherein a substantial portion of the Al metal present in the Al layer is removed by anodizing the Al layer using conventional anodization methods. As set forth, this process is used in an attempt to provide high temperature stability of the absolute humidity sensor disclosed by Kovac et al since a major portion of the Al metal left in the sensor which can become oxidized during operation or storage of high temperatures, is removed, which would otherwise affect the sensitivity and, consequently, calibration of the sensor.
However, since the Al.sub.2 O.sub.3 layer of the Kovac et al sensor is formed according to conventional anodizing methods, the Kovac et al humidity detector is incapable of providing a linear response with respect to relative humidity. Rather, the Kovac et al detector can only provide a non-liner response to absolute humidity due to the structure and density of the Al.sub.2 O.sub.3 layer resulting from the manner in which the Al.sub.2 O.sub.3 is produced, i.e., with conventional anodizing methods. Furthermore, conventional methods of anodizing utilized by Kovac et al cannot remove essentially all the Al metal and other impurities in the Al.sub.2 O.sub.3 layer, but only a substantial portion, as set forth in U.S. Pat. No. 4,143,177. Moreover, the Kovac et al device utilizes an external source to heat the substrate of the Kovac et al sensor for the purpose of driving moisture out of his sensor to prevent aging. This results, however, in an extremely slow response time of the Kovac et al sensor to changes in relative humidity.
Implementation of the sensor in the manner disclosed in application Ser. No. 92,766 entitled Device and Method of Manufacturing a Relative Humidity Sensor and Temperature Sensor filed Nov. 9, 1979 by Paul F. Bennewitz et al, now U.S. Pat. No. 4,288,755 has resulted in a detected response signal from said relative humidity sensor which varies slightly with changes in environmental temperature due to changes in output sensitivity of the sensor for changing environmental temperatures. Additionally, implementation of the sensor in high temperature environments, i.e. 50.degree. C. or more, causes slow top electrode resistance changes which decreases the calibration period of the relative humidity detector system. The calibration period is also decreased by the application of a constant d.c. biasing signal.